1. 08/09/13
B9 InheritanceB9 Inheritance
Syllabus statementsSyllabus statements

2. 9.1 Chromosomes and genes9.1 Chromosomes and genes
1. Define inheritance as the transmission of
genetic information from generation to
generation.

3. 9.1 Chromosomes and genes9.1 Chromosomes and genes
2. Define the terms:
•chromosome as a thread of DNA, made up of a
string of genes,
•gene as a length of DNA that is the unit of
heredity and codes for a specific protein. A gene
may be copied and passed on to the next
generation,
•allele as any of two or more alternative forms of
a gene.

4. 9.1 Chromosomes and genes9.1 Chromosomes and genes
3. Define the terms:
•haploid nucleus as a nucleus containing a single
set of unpaired chromosomes (e.g. sperm and
egg),
•diploid nucleus as a nucleus containing two sets
of chromosomes (e.g. in body cells).
4. Describe the inheritance of sex in humans (XX
and XY chromosomes).

5. 9.2 Cell division9.2 Cell division
1. Define mitosis as nuclear division giving rise to
genetically identical cells in which the
chromosome number is maintained by the exact
duplication of chromosomes (details of stages are
not required).
2. State the role of mitosis in growth, repair of
damaged tissues, replacement of worn out cells
and asexual reproduction.

6. 9.2 Cell division9.2 Cell division
3. Define meiosis as reduction division in which
the chromosome number is halved from diploid to
haploid (details of stages are not required).
4. State that gametes are the result of meiosis.
5. State that meiosis results in genetic variation
so the cells produced are not all genetically
identical.

7. 9.3 Monohybrid inheritance9.3 Monohybrid inheritance
1. Define the terms:
•genotype as the genetic makeup of an organism
in terms of the alleles present (e.g. Tt or GG);
•phenotype as the physical or other features of
an organism due to both its genotype and its
environment (e.g. tall plant or green seed);
•homozygous as having two identical alleles of a
particular gene (e.g. TT or gg). Two identical
homozygous individuals that breed together will
be pure-breeding;

8. 9.3 Monohybrid inheritance9.3 Monohybrid inheritance
• heterozygous as having two different alleles of
a particular gene (e.g. Tt or Gg), not pure-
breeding;
• dominant as an allele that is expressed if it is
present (e.g. T or G);
• recessive as an allele that is only expressed
when there is no dominant allele of the gene
present (e.g. t or g).

9. 9.3 Monohybrid inheritance9.3 Monohybrid inheritance
2. Calculate and predict the results of
monohybrid crosses involving 1 : 1 and 3 : 1 ratios.

10. 9.4 Variation and selection9.4 Variation and selection
1. State that continuous variation is influenced by
genes and environment, resulting in a range of
phenotypes between two extremes, e.g. height in
humans.
2. State that discontinuous variation is caused by
genes alone and results in a limited number of
distinct phenotypes with no intermediates e.g. A,
B, AB and O blood groups in humans.

11. 9.4 Variation and selection9.4 Variation and selection
3. Define mutation as a change in a gene or
chromosome.
4. Outline the effects of ionising radiation on the
rate of mutation.

12. 9.4 Variation and selection9.4 Variation and selection
5. Describe the role of artificial selection in the
production of varieties of animals and plants with
increased economic importance.
6. Describe variation and state that competition
leads to differential survival of, and reproduction
by, those organisms best fitted to the
environment.

13. 9.4 Variation and selection9.4 Variation and selection
7. Define natural selection as the greater chance
of passing on of genes by the best adapted
organisms.
8. Explain the importance of natural selection as a
possible mechanism for evolution.
9. Describe the development of strains of
antibiotic resistant bacteria as an example of
natural selection.